FIG. 9 is a diagrammatic, exploded perspective view of a conventional, standard liquid crystal display.
As shown in FIG. 9, the conventional liquid crystal display 100 comprises a polarizer 102A on the incident side, a polarizer 102B on the emergent side, and a liquid crystal cell 104.
Of these components, the polarizers 102A and 102B are so made that they selectively transmit only linearly polarized light having the plane of vibration in a predetermined direction, and are arranged in the cross nicol disposition so that the direction of vibration of the linearly polarized light transmitted by the polarizer 102A is perpendicular to that of vibration of the linearly polarized light transmitted by the polarizer 102B. The liquid crystal cell 104 comprises a large number of cells corresponding to pixels, and is placed between the polarizers 102A and 102B.
The case where the liquid crystal cell 104 in the above-described liquid crystal display 100 is of VA (Vertical Alignment) mode, in which a nematic liquid crystal having negative dielectric anisotropy is sealed in a liquid crystal cell, is now taken as an example (in the figure, a dotted line diagrammatically indicates the direction of the director of the liquid crystal). Linearly polarized light that has passed through the polarizer 102A on the incident side passes, without undergoing phase shift, through those cells in the liquid crystal cell 104 that are in the non-driven state, and is blocked by the polarizer 102B on the emergent side. On the contrary, the linearly polarized light undergoes phase shift while it passes through those cells in the liquid crystal cell 104 that are in the driven state, and the light in an amount corresponding to the amount of this phase shift passes through and emerges from the polarizer 102B on the emergent side. It is therefore possible to display the desired image on the emergent-side polarizer 102B side by properly, individually controlling the driving voltage that is applied to each cell in the liquid crystal cell 104. The liquid crystal display 100 is not limited to the above embodiment in which light is transmitted and blocked in the above-described manner, and there is also a liquid crystal display so constructed that light emerging from those cells in the liquid crystal cell 104 that are in the non-driven state passes through and emerges from the polarizer 102B on the emergent side, but that light emerging from those cells that are in the driven state is blocked by the polarizer 102B on the emergent side.
Consideration will now be given to the situation that linearly polarized light passes through the non-driven-state cells in the above-described liquid crystal cell 104 of VA mode. The liquid crystal cell 104 is birefringent, and its refractive index in the direction of thickness is different from its refractive indices in the direction of plane. Therefore, of the linearly polarized light that has passed through the polarizer 102A on the incident side, the light that has entered the liquid crystal cell 104 along the normal to the liquid crystal cell 104 passes through the liquid crystal cell 104 without undergoing phase shift, but the light that has slantingly entered the liquid crystal cell 104 in the direction deviating from the normal to the liquid crystal cell 104 undergoes phase shift while it passes through the liquid crystal cell 104 and becomes elliptically polarized light. The cause of this phenomenon is that those liquid crystalline molecules, which are vertically oriented in the liquid crystal cell 104 when the cells in the liquid crystal cell 104 of VA mode are in the non-driven state, function as a positive C plate. It is noted that the amount of phase shift that occurs for light passing through the liquid crystal cell 104 (transmitted light) is affected also by the birefringence of the liquid crystalline molecules sealed in the liquid crystal cell 104, the thickness of the liquid crystal cell 104, the wavelength of the transmitted light, and so on.
Owing to the above-described phenomenon, even when the cells in the liquid crystal cell 104 are in the non-driven state and linearly polarized light should pass through the liquid crystal cell 104 as it is and should be blocked by the polarizer 102B on the emergent side, a part of the light emerging slantingly from the liquid crystal cell 104 in the direction deviating from the normal to the liquid crystal cell 104 is to leak from the polarizer 102B on the emergent side.
For this reason, the above-described conventional liquid crystal display 100 has the problem that the display quality at the time when the displayed image is viewed obliquely from a position not on the normal to the liquid crystal cell 104 tends to be poorer than the display quality at the time when this image is viewed from a position right in front of the display (viewing angle dependency problem).
In order to improve the viewing angle dependency of the above-described conventional liquid crystal display 100, a variety of techniques have been developed up to now. Such a liquid crystal display as is described in Japanese Laid-Open Patent Publication No. 67219/1991, for example, has been known as one of these techniques. This liquid crystal display uses a retardation optical element comprising a retardation layer having a cholesteric structure (a birefringent retardation layer), where the retardation optical element is placed between a liquid-crystal cell and a polarizer in order to provide optical compensation.
In the retardation optical element having a cholesteric structure, the selective reflection wavelength given by the equation λ=nav·p (p: the helical pitch in the helical structure consisting of liquid crystalline molecules, nav: the mean refractive index on the plane perpendicular to the helical axis) is adjusted so that it is either shorter or longer than the wavelength of transmitted light, as described in Japanese Laid-Open Patent Publication No. 322223/1992, for example.
In the retardation optical element described above, linearly polarized light that has slantingly entered the retardation layer in the direction deviating from the normal to the retardation layer undergoes phase shift, while it passes through the retardation layer, to become elliptically polarized light, as in the case of the above-described liquid crystal cell. The cause of this phenomenon is that the cholesteric structure functions as a negative C plate. The amount of phase shift that occurs for light passing through the retardation layer (transmitted light) is affected also by the birefringence of the liquid crystalline molecules in the retardation layer, the thickness of the retardation layer, the wavelength of the transmitted light, and so on.
It is therefore possible to significantly improve the viewing angle dependency of conventional liquid crystal displays by using the above-described retardation optical element, if the retardation layer contained in the retardation optical element is properly designed so that the phase shift that occurs in a liquid crystal cell of VA mode functioning as a positive C plate and the phase shift that occurs in the retardation layer functioning as a negative C plate cancel each other.
The viewing angle dependency of liquid crystal displays can be improved more significantly by using a retardation layer that functions as a negative C plate (i.e., a retardation layer in which the relationships among its refractive indices Nx and Ny in the direction of plane and its refractive index Nz in the direction of thickness are Nx=Ny>Nz) and a retardation layer that functions as an A plate (i.e., a retardation layer in which the relationships among its refractive indices Nx and Ny in the direction of plane and its refractive index Nz in the direction of thickness are Nx>Ny=Nz) in combination, as described in Japanese Laid-Open Patent Publication No. 258605/1999, for example.
However, it has been found that, in the case where the above-described conventional retardation optical element (a retardation layer having a cholesteric structure, functioning as a negative C plate) is placed between a liquid crystal cell and a polarizer, although viewing angle dependency can be improved, bright-and-dark patterns, etc. can appear on the displayed image to greatly lower the display quality. In particular, it has been found that the display quality lowers drastically when a retardation layer functioning as a negative C plate and a retardation layer functioning as an A plate are, as described above, used in combination for a retardation optical element.
Conducting experiments and computer-aided simulations, the inventor has earnest studies in order to find the reason why such a retardation optical element (comprising a retardation layer functioning as a negative C plate and a retardation layer functioning as an A plate) causes the appearance of bright-and-dark patterns, etc. As a result, the inventor has finally found that this phenomenon is partly attributed to the directions of the directors of liquid crystalline molecules on the surfaces of the retardation layers.
Regarding a circularly-polarized-light-extracting optical element comprising one or more cholesteric liquid crystal layers, the inventor has already made a variety of proposals on the directions of the directors of liquid crystalline molecules on the surfaces of the liquid crystal layer(s) and also on the directions of the directors of liquid crystalline molecules in the vicinity of the interface between two neighboring liquid crystal layers (Japanese Laid-Open Patent Publication No. 189124/2002, and Japanese Patent Application No. 60392/2001 (Japanese Laid-Open Patent Publication No. 258053/2002)). However, these proposals are only for a circularly-polarized-light-extracting optical element comprising a single, cholesteric liquid crystal layer or a plurality of cholesteric liquid crystal layers that are laminated to each other, and a construction suitable for such a retardation optical element (comprising a retardation layer functioning as a negative C plate and a retardation layer functioning as an A plate) as is described above has not yet been completely made clear.